1. Field of the Invention
The present invention relates to an implantable heart stimulator.
2. Description of the Prior Art
Immediately after implantation of a heart stimulator and insertion of a heart electrode lead into the heart and attachment of a stimulation electrode carried by the head to heart tissue, the stimulation threshold, i.e. the least energy (pulse amplitude if the pulse width is constant) required to achieve heart contraction, is relatively high. After the first months following implantation of a heart stimulator the stimulation threshold eventually stabilizes at a more or less constant value in the order of some Volts (2-5 Volts).
Natural fluctuations of the stimulation threshold occur due to e.g. the activity of the patient (awake or asleep), the intake of drugs, etc. These fluctuations might be considered as more or less predictable.
Another type of fluctuation, which is considered more unpredictable, might occur when the electrode surface of the electrode lead is not in good contact with heart tissue.
U.S. Pat. No. 5,836,990 discloses a method and apparatus for determining electrode/tissue contact. The contact between an electrophysiology catheter electrode and cardiac tissue covered by blood is sensed by applying a constant voltage or current square wave signal to the electrode and then monitoring the voltage or impedance or current at the electrode before, during and after the electrode contacts the tissue. In the apparatus disclosed in U.S. Pat. No. 5,836,990 a dedicated signal is applied to the cardiac tissue in order to determine an electric parameter related to the contact between the electrode and the cardiac tissue. The electric parameter is then monitored and the contact is determined.
A drawback with the apparatus disclosed in the above patent is that it requires dedicated circuitry in order to determine the degree of contact between the electrode and the cardiac tissue.
An object of the present invention is to provide an improved implantable heart stimulator that avoids the above mentioned drawbacks with the apparatus disclosed in U.S. Pat. No. 5,836,990 and that identifies varying stimulation thresholds due to microinstability.
Microinstability is as a term used to describe how stable the electrode surface, e.g. the electrode tip, is attached to the heart tissue. If the electrode surface not is properly attached to the heart tissue, movements of the electrode might result in varying stimulation thresholds dependent on varying electrical conditions around the electrode. This in turn might result in more frequent delivery of back-up pulses which can be both unpleasant for the patient and reduce the battery capacity of the heart stimulator.
The object of the present invention thus is achieved by performing a microinstability test in order to identify varying stimulation thresholds and to increase the stimulation energy if so required to eliminate the influence of the varying threshold.
It is a further object of the invention to perform, in addition to a stimulation threshold search, a microinstability test in order to be able to adjust the stimulation energy to a level at which the effects of microinstability are reduced or eliminated.